This invention relates to a method of fabricating a polycrystalline silicon wafer used for a solar battery, a photoelectric converter, etc.
A polycrystalline silicon wafer was heretofore fabricated by a variety of methods. Generally, an ingot of prescribed shape is temporarily cast from a silicon base material, and is then sliced to fabricate a wafer. Since this conventional method takes a long time to slice the ingot and yet approx. 50% of the ingot is wasted at the time of slicing the ingot, the product of the wafer becomes expensive, and it is further impossible to fabricate a number of wafers in a mass production.
A ribbon method and a casting method are already executed as a method without slicing step for that purpose. The ribbon method has, for example, the steps of injecting molten silicon to the peripheral surface of a rotating drum and forming a ribbon-shaped wafer on the peripheral surface of the drum. According to this ribbon method, a ribbon having only a width of several millimeters can be fabricated, and it has the disadvantage that a solar battery cell of large size cannot be fabricated.
The casting method has the steps of heating a silicon base material into its molten liquid, pouring the molten silicon material in a mold prepared in response to the size of the wafer of the product, and further press molding the molten material by the movable mold to solidify the molten material. According to this casting method, the wafer of prescribed shape can be obtained simultaneously and a good result can be expected from the standpoint of its mass production, but the molten silicon base material flows from all the peripheral directions.
Thus, the growth of silicon crystalline grains is disadvantageously suppressed according to this casting method when the molten silicon material is solidified along the upper, lower and side surfaces of the casting mold. This causes the vicinity of the parts of the silicon material to be solidified in contact with the upper, lower and side surfaces of the mold to become extremely fine crystalline grains, but cannot obtain large crystalline grains. This does not satisfy the requirements of the production of large crystalline grains which are desired for a silicon wafer used for a solar battery cell. Accordingly, the photoelectric conversion efficiency of the solar battery thus obtained with the wafer is badly deteriorated to 2 to 3% as its drawbacks and disadvantages.